Prior art fuel control systems for gas turbine engines have incorporated a single control device. Such devices enable an operator to manually position the main fuel metering valve and/or other control elements in the event of a failure in the automatic positioning system. Examples of automatic fuel controls having automatic positioning systems, as well as manual controls therefore, are illustrated in U.S. Pat. Nos. 3,820,323; 4,077,203; and 4,302,931. Although such manual "back-up" controls have worked satisfactorily, the possibility remains that the entire fuel control device may become inoperable during flight (e.g. failure of both the automatic positioning control as well as the manual positioning control), resulting in engine shut down due to interruption of the fuel supply.
Although it may be recognized as desirable to provide redundant separate fuel control devices, in parallel, each capable of independently supplying the fuel required by an engine, this approach involves serious risks during the transition period. Transient disruptions in the fuel flow rate during the transition from one parallel device to another can cause engine flame out and/or overfueling which in turn can result in overtemperature or overspeed conditions and serious engine damage.